Absolute permissive block system of railway signaling



June 10, 1941. Y C STAPLES 2,244,901

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ABSOLUTE PERMISSIVE BLOCK SYSTEM-OE RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Sheet 3 l l I l I I l l l I I l L 1 1NVENTOR'I ins AJTORNEY June 10, 1941. c, E, STAPLES 2,244,901

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ABSOLUTE PERMISSIVE BLOCK SYSTEM OF RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Sheet 7 V Fig. 5a. 0W5 (Feedflackfind) 7.5 Wcmzer' End) 1 7' V 5 I+ 1&5 ?J 19 -26 I 59 KS #37 E55 47 5.9 51 55-5 5 -48 51 2 1 5 5v 55- 5s TR 62 56 v F J i 4? 44 0275 c1780 NP Maszez End) INVENTOR U awfoz .Szq vles.

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I) ma +1 Masez End) [Feed flack End] INVENTOR HIS AJI'TORNEY June 10, 1941. c. E. STAPLES ABSOLUTE PERMISSIVE BLOCK SYSTEMOF RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Sheet l0 QM e 95 a? 1 Rfl Y 7 mm M w UQWN m 7, will: QED smfiw Q m ww ln+ H i l p A SE n m 5 QMHND RED mm P Wm U m m m @N m5 Q o I @w wa mm mm Q E QM QR A E M mm Y N A fi m 4 mm wkmwo $3 1% W 01 k @m m./\.r+ N. WM. N H N k 2 Km June 10, 1941. c. E. STAPLES ABSOLUTE PERMISSIVE BLOCK SYSTEM OF RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Sheet ll I QBQKRNW KQMWG I IIKBED .KEWU LIAEQQU kwwwhgw Al W QB wmm w mwzmowwikt 2 wmw wmamwl Cpawfo Szzydes.

H15 AITITORNEY June 10, 1941. c. E. STAPLES 2,244,901 ABSOLUTE PERMISSIVE BLOCK SYSTEM OF RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Shear, 13

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c E. STA LES ABSOLUTE PERMISSIVE BLOCK SYSTEM OF-- RAILWAY SIGNALING 15 Sheets-Sheet 14 Filed Jan. 26, 1940 ar le;

June 1941- c. E. STAPLES- ABSOLUTE PERMISSIVE BLOCK SYSTEM OF RAILWAY SIGNALING Filed Jan. 26, 1940 15 Sheets-Sheet 15 w RN 5% E .i 0 m Wm 51 r M Q g m R D n; @QN $5 $5 W2 m m m i H. mm. 7mm I U n m 5s & NM m Patented June 10, 1941 ABSOLUTE PERIVIISSIVE BLOCK SYSTEM OF RAILWAY SIGNALING Crawford E. Staples, Wilkinsburg, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application January 26, 1940, Serial No. 315,801

39 Claims.

My invention relates to absolute permissive block (A. P. B.) systems of railway'signaling and it has special reference to systems of this character wherein all major functions of signal control are performed without the aid of line wires and by means of coded energy that is transmitted through the track rails.

Generally stated, the object of my invention is to improve certain features of non-line-wire A. P. B, systems that utilize track circuits which operate on the coded feed back principle of Frank H. Nicholson Patent No. 2,021,944.

A more specific object is to improve the design and broaden the utility of track and other circuits of the coded feed back type.

Another object is to extend the number of signal control functions which such track circuits are capable of performing in systems of two-direction signaling.

An added object is to utilize these extended functions in such manner that each train coming into and passing through the two-direction running stretch automatically sets up its own directional protection and provides its own following protection wholly by means of the coded energy which the track circuits transmit.

An additional object is to provide for the ap proach energization of each and every signal in the non-line-wire A. P. B. system and in other respects to lower the power consumption of the system apparatus.

A further object is to reduce the quantity of the decoding and signal control apparatus which A. P. B. systems of the above referred to 11011-7 line-wire type require. v

Still further objects are to provide for the nonline-wire A. P. B. control of either or both wayside signals and train-carried cab signals; to readily extend the number of indications which the controlled signals display; to adapt the improved A. P. B. system to automatic signal blocks which both do and do not contain cut sections; to make improved provision for controlling highway crossing signals which the named blocks may embrace; and to retain all of the advan tages of continuously coded track circuits.

In practicing my invention I attain the above and other objects and advantages by equipping each of the track sections in the two-direction running stretch with track circuit facilities of the coded feed back type previously referred to; providing means that are responsive to the direction of train movement through the stretch and that at all times cause the recurring pulses of master code energy to be supplied to the train exit ends of the stretch sections; imparting normal polarity to the off" period pulses of feed back energy under vacant conditions of the stretch and using those normal polarity pulses to hold at proceed the headblock signals which guard the extreme ends of the stretch; reversing the polarity of the feed back pulses when a train comes into the stretch and using those reverse polarity pulses to put the opposing headblock signal at stop and thereby establish directional protection for the train; using the feed back pulses of both normal;and reverse polarity to keep the system signals deenergized at all times except when indications must be displayed to an approaching train; and using the recurring pulses of master code energy to control the signals behind each train in customary automatic block manner whereby to provide following protection for the train.

I shall describe one representative form of A. P. B. signaling apparatus embodying my invention and shall then point out the novel features thereof in claims. This illustrative embodiment, together with supplemental showings of certain modified portions thereofQis disclosed in the accompanying drawings in which:

Figs. 1a to 16, inclusive, are diagrammatic views which when placed end to end in the order named represent a stretch of two-direction running track that is equipped with A. P. B. signaling apparatus which embodies the improvements of my invention;

Fig. 2 is a diagrammatic showing of master" and feed back codes which are suitable for controlling the apparatus of Figs. let-e; 7

Figs. 3 and 4 represent facilities which may be used to supply energy to certain portions of that apparatus;

Figs. 5 to 8, inclusive, are simplified showings of the coded feed back track circuits which are identified with the several track sections of Figs. 1a,-e;

Figs. 9 and 10 are single line representations of the track stretch of Figs. la-e showing the trackway codes and thewayside signal indications that are effective under various traffic conditions of the stretch;

Figs. 11 to 14 show alternative forms which certain of the relay control circuits of Fig. 1

may take;

Fig. 15 is a representation of cab signaling a p paratus which may be controlled by the trackway equipment of the preceding figures;

Fig. 16 shows an adaptation of the A. P. B.

system apparatus of Fig. l to the control of wayside signals of the searchlight type; and

Fig. 17 shows an alternative arrangement for controlling certain of the signals in the A. P. B. system.

In the several views of the drawings, like reference characters designate corresponding parts. To facilitate the description of the various circuit diagrams of these views, it will be assumed that the left end thereof is west and the right end is east. Hence train movements in the direction of from left to right will be spoken of as eastbound and movements in the direction of' from right to left will be spoken of as west- .III- IIIa and IIIaIV and the rails of each of these sections form a part of a coded feed back track circuit combination which will be described presently.

Positioned at the locations I, II, III and IV of track section division are wayside signals SE which are arranged to govern eastbound trafiic through the track stretch and other wayside signals SW which similarly are arranged to govern the passage of westbound trains over the track. These signals may, of course, be of any suitable form such as of the color light type of Fig. l,

, the searchlight type of Fig. 16, or any other equivalent type not shown. When of the color light type of Fig. 1, each of the named signals includes three lamps G, Y and R which when selectively lighted respectively display the indications of green or clear, yellow or approach,

and red or stop.

The signal SEI at the west end I of the represented stretch of A. P. B. track governs the entry of eastbound traific into that stretch while the signal SWIVat the opposite or east end IV similarly controls the entry of westbound traffic into the stretch. To distinguish the two just named headblock signals from the remaining signals of the system, they are provided with a fourth lamp which is constantly illuminated in the manner represented at M.

The location Illa of, the composite diagram of Fig. 1 marks a cut in the main signal block III-IV and hence it has no wayside signal associated with it. Such a cut may, of course, be occasioned by excessive block length, by an intersecting highway such as is indicated at 4, or for any other reason. At the intersection which is represented in Fig. 1d, a highway crossing signal XS is installed in the usual way.

The. particular stretch of two-direction running track that is represented in Figs. la-e thus includes (between the end or headblock signal locations I and IV) two intermediate locations 11 and III at which wayside signals SE and SW are provided and one cut section location IIIa with which a highway crossing signal XS is associated. Alternatively, of course, the stretchmay include either 'a greater or lesser number of these intermediate and cut section locations. Moreover, all of the intermediate 10- 'and 10. This form of control permits two more trains to follow each other through portions of the single track that extend between passing fsidings and at the same time prevents a train from entering the single track stretch when that stretch is occupied by another train moving in the opposite direction. In the represented scheme of my invention, moreover, this A. P. B. control of the system signals is effected without the aid of line wires and wholly by meansof coded energy which is' transmitted through the track rails. 1 1

Coded feed-back track: circuits and reversing means the refer This novel and advantageous form of A. P.-,B. control is made possible by operating two-:coded track circuits simultaneously on each of the .sections I-II, II-III, III-IIIa and IIIa.-IV into which the track I-2 of Fig.1 is divided; One of these track circuits transmits recurring pulses of master code energy over the-section rails in one direction and it will-be'referred toas the first or master track circuit; the other transmits off period pulses of feed back energy over the section rails in the opposite direction and it will be referred to asthe second or feed back track circuit. To designate both of these simultaneously operating track circuits, use will be made of the expression -coded feed-back track circuit combination.

One elementary form of such a combination is disclosed and claimed in the before referred to Nicholson Patent No. 2,021,944.: In the coded feedback track circuit combinationof that patent the direction of transmission of the master code pulses is always the same; the direction of transmission of the feed back pulses likewise never changes; and the character of these feed back pulses is never alteredin a-selectivcly distinctive way. y I I As utilized by the non-line-wire A. P.-B.-systern of my invention the coded feed back track circuits now about to be described possess-a number of added refinements and they are-,moreover, novelly arranged to have-all three of the just named factors variously change in response to the passage of trains through the circuited sections of the two-direction running track. In the case of the first two named factors (directions of master and "feed back energy transmission) these changes are effected by trafiic directional relays TD that, in the manner shown, are installed at all of the intermediate locations (II, III and HM) in the system.

At times these traffic directional relays TD hold their contacts in the right-biased position and they then set up the several coded feed back track circuit combinations of the stretch in the manner which is shown in Figs. la-e, 5a, 6a and 7a; at other times, however, all of the relays TD shift their contacts to the left and they then transfer the track circuit set-ups to the reversed relation which is represented in Figs. 5b, 6b and 7b. The set-up first named is in effect when the stretch is conditioned for the passage of eastbound trains while the reversed set-up is in effect when the stretch is conditioned for the passageof westbound trains.

During (the first or eastbound set-up of the track circuits the master code energy is trans mitted in the direction of from east to west (or towards the approaching eastbound trafiic) while the feed back energy is transmitted in the opposite or west-to-east direction. This relation is most clearly represented in Figs. 941- and 10d. It, of course, requires that the master energy supply facilities be located at the east end of each section and that the feed back energy supply facilities be located at the opposite or west end. Such a positioning is shown in Figs. a, 6a, 7a. and 8.

During the reversed or westbound set-up of the track circuits the master-code energy is transmitted in the direction of from west to east (or towards the approaching westbound traffic) while the feed back energy is transmitted in the opposite or east-to-west direction. This relation is most clearly represented in Figs. a-c and 9d. It requires that the master energy supply facilities be located at the west end of the section and that the feed back energy supply facilities be located at the opposite or east end. Such a positioning is shown in Figs. 5b, 6b and 71).

As each of the coded feed back track circuit combinations which the simplified diagrams of Figs. 5, 6, 7 and 8 represent is distinctive in certain of its details, these drawing views will be considered individually. As has already become evident, Figs. Sal-b show the track circuits which are identified with track section I-II of the complete stretch of Fig. 1; Figs. 612-?) show the track circuits for section II-I1I; Figs. Ta-b show the circuits for section III-IV; and Fig. 8 shows the track circuits for an end section which spans one of the passing sidings PS.

Looking first at the track circuits for section 1-11, the "eastbound set-up thereof is represented in Fig. 5a and the "westbound set-up in Fig. 5b. These two set-ups may be distinguished from each other by the trafiic direction arrows (left-to-right for eastbound and right-to-left for westbound) and also by the interchanged relation of the master and feed back ends.

In each of the two set-ups both of the before referred to master and feed back track circuits include the section rails I and 2. In the eastbound set-up of Fig. 5a. the master circuit further includes an energy source TB and a code repeating relay CR installed at the section east end II and a code following track relay NKE (together with a companion relay RKE) installed at the section west end I; in this same eastbound set-up, likewise, the feed back circuit further includes an energy source KB and a rail connection transfer device CRW located at the section west end and a pair of polarized track relays RK and NK installed at the section east end.

The just mentioned energy sources TB and KB preferably are direct current track batteries and may be of any suitable type, such as primary, storage or the like. Alternatively, of course, these direct current sources also may take the form of rectifier units (not shown) which convert alternating current energy from a commercial supply source into the required direct current.

The code repeating relay CR for the master track circuit of Fig. 5a functions through a contact 6 thereof; (1) periodically to connect the master track battery TB in energy supplying relation with the section rails for the purpose of producing one or the other of the two master codes of Fig. 2; and (2) to connect the feed back circuit track relays RK and NK in energy receiving relation with those rails during each off period of the thus supplied master code. Current for operating this relay CR is supplied to the winding thereof by way of a circuit that includesone or the other of a pair of coding contacts I80 and 15 selected over a contact I of a relay H (later to be described).

In the particular arrangement shown, these coding contacts I80 and 75 form parts of two separate code transmitters CT|80 and CT'I5 each of which actuates the contact at its distinctive preselected speed. For purposes of explanation it will be assumed that the speed of up and down movement effected by contact I80 is such as to produce the 180 pulse per minute master clear code of Fig. 2 and that the speed of actuation of contact 15 is such as to produce the 15 pulse perminute "master approach code of Fig. 2. Instead of forming parts of the represented separate coder units the two contacts I80 and 15 may, of course, be included in a single transmitting device having the usual common driving mechanism which actuates the two contacts at the required different distinctive speeds.

The master circuit track relay NKE (at the section west end) is a code following device which responds only to positive or normal polarity energy and which has a control winding 8 that is connected in energy receiving relation with the section rails over the back point of a contact 9 of the before named device CRW. This relay NKE is provided with contacts II and i2 which pick up during each on period of master code energy that the control winding receives and which release during each master code 01f period. Serially included in the control winding connection of device NKE is a companion code following relay RKE which responds only to negative or reverse polarity energy and which performs a function later to be described in connection with Fig. 5b.

Coding of the energy which the "feed back track battery KB (at the section west end) supplies to the section rails is performed by back contact H of the'master circuit track relay NKE aided by contact 9 of the just mentioned device CRW. That device receives pick-up current over front contact l2 of relayNKE and has a period of release delay which is slightly less than the off period length of the fastest master code to which relay NKE is called upon to respond. In a manner which will presently become apparent, the just named contacts 9 and H transmit feed back energy to the section rails during the off periods of the master trackway code that is received by relay NKE.

Each pulse of this "feed back energy constitutes an on period in one of the feed back codes of Fig. 2. From an inspection of that figure it will be seen that the on periods of each of the there shown feed back codes coincide with the off periods of the master code which is represented immediately thereabove, and vice versa. This relation allows the master and the feed back track circuits simultaneously to operate over the same section rails without interfering with each other.

The feed back circuit track relay NK (at the section east end) also is a code following device which responds only to positive or "normal polarity energy and which has a control winding 8 that during the off periods of the before described master trackway code is connected in energy receiving relation with the section rails over the back point of contact 6 of the before described code repeating relay CR. Serially included in the represented connections is a companion code following relay RK which responds only to negative or reverse polarity energy and which performs a function later to be described in connection with Fig. 51).

As already indicated, the feed back circuit track relays NK and RK (at the section east end are both disconnected from the section rails (under the action of contact 6 of device CR) during the on periods of the there supplied master code. This disconnection prevents the fmaster track circuit energy from reaching those relays and thus renders them responsive only to energy that is supplied to the rails from the feed back track battery KB at the section west end.

As will become more evident presently, the master circuit track relay NKE (and device RKE also) at the section west end is similarly disconnected from the section rails (under the action of contact 9 of device CRW) during the on periods of the there supplied feed back code and in that manner those relays are rendered responsive only to energy that is supplied to the rails from the master track battery TB at the section east end.

In operation of those basic portions of the master and feed back track circuits for section III which are shown in Fig. 5a (for the eastbound set-up of the section), the code repeating relay CR at the east end of the section acts as a master device in that it sets the rate at which the pulses of both the master and the feed back trackway codes recur. This rate, in turn, is determined by which of the coding contacts and I80 is included (under the selecting action of relay H) in the driving circuit for the code repeating relay CR.

As already indicated,both of these coding con tacts move (each at a distinctive rate) between an upper position and a lower position. When in the upper position the selected or active contact completes for the code repeating relay CR a pick-up circuit which may be traced from the positive terminal of a suitable supply source (such as a local control battery of the type shown at CB-in Fig. 3), through one or the other of coding contacts I80 and 15, contact I of relay H, conductor l3 and the winding of relay CR, back to the negative terminal of the control source.

By picking up contact 6 of relay CR, each of these completions constitutes an on period of one of the master codes of Fig. 2 and causes the track battery TB to energize the master circuit track relay NKE (at the section west end) over a circuit which extends from the positive terminal of that battery through conductor 14, the secondary of a transformer 15, conductors l6 and I1, a contact l8 of the traflic directional relay TD, conductor [9, track rail 2, conductors 2| and 22, the control winding 8 of relay NKE, relay RKE, conductor 23, back contact 9 of relay CRW, conductor 25, track rail 1, conductor 25, contact 21 of relay TD, conductor 28, front contact 6 of device CR and a current limiting impedance 29 back to the negative terminal of battery TB. l

in responding to each of the thus supplied energizing pulses, relay NKE picks up and causes contact I l to disconnect the feed back battery KB from the section rails and thereby produce a feed back code 01f period. At the same time contact I2 of relay NKE energizes device CRW over a circuit extending from the positive terminal of a local supply source (which again may take the form of a control battery such as is shown at CB in. Fig. 3), through front contact 12, conductor 3| and the winding of device CRW back to the negative terminal of the control source.

Device CRW now picks up contact 9 and thereby transfers the connection of track circuit conductor 25 from conductor 23 to a conductor 32. Even under this new condition, however, relay NKE continues its energy receiving connection with the track rails but by way of a by-pass which includes a conductor 33, front contact ll of relay NKE, conductor 32 and front contact 9 of device CRW. This by-pass path is established early in the master code on period (contact 9 included therein picks up with sufiicient rapidity to insure that relay NKE will not falsely release during the transfer of its supply connection from the back point of contact 9 to conductors 33 and 32) and once set up the by-pass path continues uninterruptedly until the .end of the master code on period during which it occurred.

In going to the lower position at the end of the master code on period, the active coding contact (I or 15 code) at the east end of the section interrupts the energizing circuit for code repeating relay CR, that action releases contact 5 of the repeating relay, and that release marks the beginning of a master code off period. During each of those off periods, the just named contact 6 connects the feed back circuit track relays NK and RK in energizing receiving relation with the track rails.

At the same time (that is, during each master code off period) the master circuit track relay NKE at the section west end becomes deenergized and it releases in the usual manner. That release, in turn, marks the beginning of an on period of one of the feed back codes of Fig. 2.

Incident thereto there is completed (at contact H) a circuit over which battery KB at the feed back end of the section energizes the track relays NK and RK at the east section end. This circuit may be traced from the positive terminal of track battery KB through conductor 34, the current limiting impedance 35, conductor 2|, track rail 2, conductor l9, contact N3 of relay TD, conductor I1, control winding 8 of relay NK, relay RK, conductor 36, back contact 6 of device CR, conductor 28, contact 21 of relay TD, conductor 26, track rail l, conductor 25, front contact 9 of device CRW,conductor 32, back contact H of relay NKE, conductor 31, the secondary of a transformer 15 and conductor 38 back to the negative terminal of battery KB.

Each of the above transmitted pulses of feed back energy is efiective for .picking up relay NK but does not produce any response on the part of the companion device RK (due to the before explained polarized characteristics of these feed back circuit devices). At the opposite or west end of the section, the rail disconnecting action of the now released contact ll of the relay NKE prevents that relay from responding to the feed back code energy with which the section rails are supplied.

During the entire length of this pulse, of

circuit. for this device CRW is interrupted at contact l2 of relay NKE at the beginning of the master code off period. This interruption starts the release timing and thus causes each pulse of feed back code energy to have a length which is the same as that of all other pulses of that energy and the magnitude of which is indicated at ,f in Fig. 2. This action, of course, explains why the feed back energy pulses which recur at the slow rate of '75 times per minute are no longer than the feed back energy pulses which recur at the higher rate of 180 pulses per minute.

Inreleasing as just described, device CRW terminates the feedback code on period and conditions the master circuit track relay NKE for a reception of and a response to a succeeding pulse of master code energy. That pulse is supplied to the section rails when the code repeating relay CR at the section east end again moves its contact 6 to the picked up position.

,When that happens the just described cycle of two-direction coded track circuit operation starts torepeat itself. In the manner just outlined,

therefore, the master and the feed back track circuits operate over the same section rails simultaneously and without interfering with each other. This lack of interference, as has been stated, results from the fact that each on period of the feed back code is supplied during an off period of'the master code and vice versa.

The coded feed back track circuits which have just been described for track section I-II operate under conditions of the A. P. B. system of Fig. 1 wherein trafilc movement through the section in the eastbound direction is authorized. As has been seen, master code energy then is supplied to the east end II of the section and feed back energy then is supplied to the west end I thereof.

For system conditions wherein trafiic movements in the opposite or westbound direction are authorized the set-up of the track circuit for section I--II is modified to the extent indicated in Fig. 51). As previously pointed out, this modification is effected by the trafiic directional relay TD through a shift of its contacts l8 and 21 from the right-hand position represented in Fig. a. to the left-hand position represented in Fig. 517. 'A later portion of the specification describes the manner in which this shift of traffic directional relay contacts is produced.

When made, the effect is to transfer the master end of'the coded feed back track circuits for section I-II from the east end II to the west end I of the section and to substitute at the east end II the feed back end equipment of Fig. 51) for the master end equipment which is shown in Fig. 5a.

In the westbound trafiic set-up of Fig. 5b, the battery KB at the west location I constitutes the source of master code energy; device CRW at the same location serves to code this energy by periodically connecting battery KB in energy supplying relation withthe section rails under the control of one or the other of a pair of code transmitters CTi and CT'I5 selected over a contact 39 of a relay NP; and the thus coded masterenergy is transmitted eastwardly over the section rails to a code following track relay TR at the section east end II.

This track relay TR may be of conventional code following'design and in the particular arrangement shown it receives the recurring pulses ofmaster code energy over the back point of a contact 4! of an impulse relay KR... That impulse relay, in turn, likewise forms a part of the substituted feed back end facilities at the east location II. Also comprised by the same facilities are a source of feed back energy represented as a track batteryKB and a relay KS which selects the polarity of the energy which this battery supplies to the track rails during the master code off periods under the control of relays KR and TR.

The function of the impulse relay KR is to transfer the connection of the track rails I and 2 from the track relay TR to battery KB during each off period of the master code energy which is received by relay TR. Pick-up current is supplied to relay KR over the front point of a contact 42 of relay TR during each on period of the received master code. During the off periods of that code the impulse relay is deenergized and contact 4| thereof then occupies the lowermost position (shown in full lines) in which it connects the operating winding of the track relay TR directly across the section rails. As in the case of device CRW which was described in connection with Fig. 5a, this relay KR has a release delay period which is slightly less than the shortest off period length of any of the master codes which the system employs.

In picking up in response to each received pulse of master code the track relay TR completes (at contact 42) for relay KR an energizing circuit which may be traced from the positive terminal of battery KB through conductor 43, the winding of relay KR, conductor 44, front contact 42 and conductor 45 back to the negative terminal of battery KB. This energization causes relay KR to move contact 4| upwardly into the picked up position (shown dotted) wherein it disconnects the track rails from relay TR. This disconnection, however, is now bridged by a contact 46 of relay TR over the front point of which the winding of that relay continues to be connected with the rails during the full continuance of the energy pulse.

At the end of that pulse relay TR releases and contact 46 now breaks the connection of its winding with the track rails. At the same time contact 42 connects those rails in energy receiving relation with the battery KB at location II by way of a circuit which includes front contact 4| of relay KR. That relay having the before-described releasing characteristics, contact M is released before the beginning of the succeeding on period of the received master code.

If the polarity selector relay KS is picked up as shown, contacts 4'! and 48 thereof establish 'a circuit over which the battery KB at location II supplies positive or normal polarity energy to the section rails during each off period of the received. master code. If, however, the contacts 41 and 48 of relay KS are released, the polarity connections of battery KB are reversed and in that event the battery supplies the section rails with feed back energy of negative or re- 

